1. Field of the Invention
This invention relates to an optical scanning device of the compact slit exposure type in which an original and a lens are fixed and which has a scanning system on the image field side.
2. Description of the Prior Art
Copying machines or the like of the slit exposure scanning type have heretofore been classified into a type which has a scanning system on the object field side and a type which has a scanning system on the image field side. The object field side is the area in the optical path from the original to the lens and the image field side is the area from the lens to the photosensitive medium.
As examples of the former, there are Japanese Patent Publication No. 6647/1964 and Japanese Utility Model Publication No. 6456/1970, and as examples of the latter, there are Japanese Patent Publication No. 30013/1971, Japanese Utility Model Publication No. 13075/1971, Japanese Patent Publication No. 10259/1968 and Japanese Laid-open Patent Application No. 102041/1978. Generally, where there is a scanning system on the object field side, it is necessary to change the movement velocity of the scanning system in accordance with the copying magnification, but where there is a scanning system on the image field side, the movement velocity of the scanning system can be made constant irrespective of the copying magnification. Incidentally, Japanese Patent Publication No. 30013/1971 which is a prior art example in which there is a scanning system on the image field side has suffered from a problem that the vicinity in which the principal ray is coincident with the optic axis cannot be used and therefore the use of a lens having a wide angle of view is required. Japanese Utility Model Publication No. 13075/1971, Japanese Patent Publication No. 10259/1968 and Japanese Patent Publication No. 13474/1971 have included translational movement as well as rotational movement and this has led to complication of the mechanism. Japanese Laid-open Patent Application No. 10204/1978 discloses that a lens whose maximum angle of view with respect to the optic axis is about half that of Japanese Patent Publication No. 30013/1971 is sufficient and the scanning system on the image field side should have only rectilinear movement. However, it has still suffered from the problem that because the reflection points of two mirrors constituting the image field side scanning system change from moment to moment during scanning, the effective portion of the mirrors must be made large and a correspondingly greater dead space in scanning must be secured resulting in increased weight which is disadvantageous for the scanning.